Mining through the houses was not sufficient. The greater streets divided the city into a number of small districts, the group of dwellings in each of which forming a separate stronghold. To cross these streets it was necessary to construct underground galleries, or build traverses, since a Spanish battery raked each street, and each house had to be fought for and taken separately. While the Spaniards held the convents and churches the capture of the houses by the French was of little service to them, the defenders making sudden and successful sallies from these strong buildings, and countermining their enemies, their numbers and perseverance often frustrating the superior skill of the French. The latter, therefore, directed their attacks upon these buildings, mining and destroying many of them. On the other hand, the defenders saturated with rosin and pitch the timbers of the buildings they could no longer hold, and interposed a barrier of fire between themselves and their assailants which often delayed them for several days. Step by step, inch by inch, the French made their way forward, complete destruction alone enabling them to advance. The fighting was incessant. The explosion of mines, the crash of falling buildings, the roar of cannon and musketry, the shouts of the combatants continually filled the air, while a cloud of smoke and dust hung constantly over the city as the terrible scene of warfare continued day after day. By the 17th of February the Cosso was reached and passed. But the French soldiers had become deeply discouraged by their fifty days of unremitting labor and battle, fighting above and beneath the earth, facing an enemy as bold as themselves and much more numerous, and with half the city still to be conquered. Only the obstinate determination of Marshal Lannes kept them to their work. By his orders a general assault was made on the 18th. Under the university, a large building in the Cosso, mines containing three thousand pounds of powder were exploded, the walls falling with a terrific crash. Meanwhile, fifty pieces of artillery were playing on the side of the Ebro, where the great convent of St. Lazar was breached and taken, two thousand men being here cut off from the city. On the 19th other mines were exploded, and on the 20th six great mines under the Cosso, loaded with thousands of pounds of powder, whose explosion would have caused immense destruction, were ready for the match, when an offer to surrender brought the terrible struggle to an end.